A pharmacological screen for `inactivating antagonists' at an essential parasite GPCR. PROJECT SUMMARY: Over a third of the world's population is infected with parasitic worms. One of the most burdensome infections underpins the neglected tropical disease schistosomiasis (Bilharzia) caused by parasitic flatworms of the genus Schistosoma, which afflicts ~200 million people worldwide. The mainstay pharmacotherapy for schistosomiasis, and several other parasitic helminth infections, is the drug praziquantel (PZQ). However, several features of PZQ are less than ideal. These include an inability of PZQ to kill all stages of the parasitic life cycle, clinical reliance on PZQ as a monotherapy in light of sub-optimal cure rates and field reports of PZQ resistance, as well as an inability to improve upon the PZQ pharmacophore or define mechanistically how this drug works. Consequently there is a need to develop next generation anthelmintics, ideally active against a broad spectrum of PZQ-sensitive helminth parasites. Our team has brought new insight to this problem from a serendipitous basic science discovery. Using regenerative screens in planarian flatworms, we have been able to dissect the pathways engaged by PZQ in vivo and our work has culminated in the identification of a parasite serotonergic G protein coupled receptor (GPCR, Sm.5HTR7) that is abundant, conserved and critical for helminth regeneration, mobility and viability. As GPCRs have proved imminently druggable targets (1/3rd of currently approved medications), the purpose of this proposal is to perform the first high throughput screen of any flatworm GPCR to discover parasite-selective ligands. To do this we have successfully optimized heterologous expression of Sm.5HTR7 and miniaturized a real time biosensor assay for monitoring Sm.5HTR7 activity. Pilot screening data has revealed that a subset of Sm.5HTR7 antagonists cause a persistent inaction of receptor signaling and parasite paralysis that persists for days after ligand removal. This curious piece of receptor phenomenology ? where a single drug dose effects a `pharmacological knockout' of receptor function ? is an ideal property for single dose clinical therapy in disadvantaged healthcare scenarios and enhances the appeal of a drug screen of this receptor. Our goal therefore is to characterize the pharmacological profile of Sm.5HTR7 through the following activities. AIM 1: to perform an unbiased drug screen (20,500 compounds) to identify `inactivating antagonists' at this receptor, and as a foil to this unbiased screen, AIM 2: to explore the SAR properties of a single class of serotonergic ligands (dimethoxyisoquinoline containing compounds) that are suggested by pilot screening data (~250 compounds) to exhibit high affinity and selectivity for Sm.5HTR7 If successful, these activities will provide new leads for subsequent development of novel antiparasitic agents.